Machine tool and bed structure thereof

ABSTRACT

A machine tool includes X-axis, Y-axis, and Z-axis moving units for producing relative movements between a tool and a workpiece; a C-axis drive unit for rotating the workpiece about a C-axis parallel to the Z-axis; and a B-axis turning unit for turning the tool about a B-axis parallel to the Y-axis. The tool is disposed in such a manner that a machining point of the tool coincides with the B-axis. The moving units, the drive unit, and the turning unit are controlled in such a manner that a work point of the workpiece coincides with the machining point of the tool. The bed is formed through casting and has a hollow structure and a hole as cast; and a cover is provided to cover the hole as cast in order to close the interior of the bed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a machine tool for precision machiningof a workpiece, and to a bed structure of such a precision machine tool.

2. Description of the Related Art

A conventional precision machine tool is disclosed in, for example,Japanese Patent Application Laid-Open (kokai) No. 10-151534. As shown inFIG. 1A, in the disclosed machine tool, a machining unit, which includesa tool T provided on a main spindle 204 having a horizontal rotationaxis, is provided on a Z-axis unit 203 (movable along a horizontalZ-axis). The Z-axis unit 203 is supported on an X-axis unit 202 (movablealong a horizontal X-axis perpendicular to the Z-axis), which isdisposed on a bed 201.

A C-axis unit 207 having a horizontal C-axis is disposed in oppositionto the main spindle 204. The C-axis unit 207 supports a workpiece W forrotation about a horizontal rotational axis. The C-axis unit 207 issupported on a B-axis unit 209 (rotatable about a vertical B-axis),which is supported on a Y-axis unit 210 (movable along a verticalY-axis), which is disposed on the bed 201.

A point of the workplace W to be machined (hereinafter referred to as a“work point”) is moved or indexed to a predetermined position by meansof the C-axis unit 207, the B-axis unit 209, and the Y-axis unit 210,whereas a machining point of a tip end of the tool T is moved or indexedto a predetermined position by means of the X-axis unit 202 and theZ-axis unit 203, whereby the work point of the workpiece W is machined(cut or ground) by the tool T at its machining point.

In the conventional machine tool, the position of the work point of theworkplace W, which is represented by “A” in FIG. 1A (overall frontview), is separated by a “distance Lbw” from the B-axis. Therefore, ifan error α is generated as shown in FIG. 1B (partial plan view) when theB-axis unit 209 is rotated by an angle θ from a position (indicated bybroken lines) at which the C-axis coincides with the Z-axis, in order toindex the work point, the work point deviates from its theoreticalposition “A(θ)” to a position “A(θ+α).” When the tool T is moved towardthe position “A(θ),” which deviates from the actual position “A(θ+α),”the tool T machines the position “A(θ),” although the position to bemachined at that time is “A(θ+α).” Such an error becomes remarkable asthe “distance Lbw” increases. Further, in addition to the error involvedin position indexing, an error stemming from a positioning deviation atthe time of B-axis stoppage becomes remarkable as the “distance Lbw”increases.

Moreover, in the conventional machine tool, as shown in FIG. 1C (partialfront view), the position “A” of the work point is separated by a“distance Lyw” from the Y-axis. Therefore, when a ram 217 (movablemember) of the Y-axis unit is vertically moved from a position at whichthe position “A” of the work point coincides with the tip end of thetool T, in order to machine the work point A, vertical forces Fu and Fdstemming from machining resistance are applied to the work point A. Theram 217 is held by a nut 221 in screw-engagement with a ball screw 220.Stemming from the “distance Lyw” and the “forces Fu and Fd,” a moment isgenerated (an unnecessary stress acts on the nut 221 in a direction notcoinciding with the Y-axis), whereby the ram 217 may incline as shown onthe right side in FIG. 1C. When an “error β” is generated stemming fromthe inclination, the work point deviates from its theoretical position“A” to a position “A(β).” When the tool T is held at a heightcorresponding to that of the position “A,” which deviates from theactual position “A(β),” the tool T machines the position “A,” althoughthe position to be machined at that time is “A(β).” Such an errorbecomes remarkable as the “distance Lyw” increases.

Influence of these errors is at a level which can be ignored in machinetools which perform ordinary machining. However, in precision machinetools which perform machining with very high accuracy on the order ofseveral hundreds to several tens of nanometers, influence of such errorsis large, and such errors must be suppressed.

Incidentally, a bed used in a precision machine tool such as a grindingmachine is generally formed by casting. In general, such a bed is castto have a hollow structure in such a manner that the bed is reinforcedby integrally formed ribs arranged in a grid pattern. Further, aplurality of holes as cast (hereinafter referred to as “cast holes”)penetrate the side and bottom walls of the bed. The reason why the bedis cast to have a hollow, rib-reinforced structure is to reduce theweight of the bed and the influence of long-term distortion of thematerial. The cast holes cannot be eliminated, because they areessential for casting a bed having a hollow, rib-reinforced structure.

In some cases, instead of a cast bed, a bed formed of stone such asgranite is used in a super-precision machine tool which must machineoptical components or the like with very high machining accuracy. Such abed formed of stone such as granite has characteristics such that thebed exhibits smaller long-term changes in material properties and alarger heat capacity as compared with the case of cast beds, andgenerally has a solid structure.

The conventional cast bed is prone to receive-Influence of outside airtemperature, because the inner structure of the bed is exposed to theoutside air through the cast holes, and the area of contact with theoutside air is larger than in a case of a bed having a solid structure.

In general, when an object has a temperature difference with respect tooutside air temperature, the time from exposure to outside airtemperature until the object attains the same temperature as the outsideair temperature decreases as the ratio of surface area S to volume V;i.e., S/V, increases. FIG. 14 shows results of calculation for obtainingtemperature changes of three objects which have the same volume and thesame temperature difference with respect to outside air temperature, buthave different surface areas. These three objects are formed of the samematerial (gray cast iron), and the calculation for each object wasperformed for the case where the initial temperature is 25° C., and theambient temperature is 20° C. FIG. 14 shows that a spherical object,having the smallest S/V value, takes the longest time to attain theoutside air temperature, and that the time required to attain theoutside air temperature decreases as the S/V value increases. In otherwords, influence of outside air temperature increases as the S/V valueincreases.

Since the conventional cast bed has a hollow, rib-reinforced structure,the bed has an S/V value greater than that of a bed having a solidstructure. Therefore, the bed temperature is prone to change as theoutside air temperature changes, and affects structures mounted on thebed; specifically, slide surfaces, the tool spindle, and the workplacespindle, whereby an error is produced in the positional relation betweena workplace and a tool. As a result, machining accuracy fluctuates inthe course of long-term machining.

The above-described problem exerts considerable influence not only on amachine tool disposed in a place, such as an ordinary plant, where theoutside air temperature changes greatly, but also on a machine tool,such as a super precision machine tool, which is placed in athermostatic room, whose interior temperature is controlled to a settemperature ±1° C. and which is required to provide very high machiningaccuracy.

Meanwhile, the conventional bed formed of stone such as granite has alarger heat capacity as compared with the case of cast beds, and has asmaller area of contact with the outside air, because it assumes theshape of a solid rectangular parallelepiped. Therefore, the conventionalbed formed of stone such as granite has an advantage in that thetemperature of the bed is unlikely to follow changes in the outside airtemperature, and the bed enables machining with high accuracy. However,the granite is more expensive than a casting, and the degree of freedomin design is low, because machining of granite is difficult.

SUMMARY OF THE INVENTION

In view of the foregoing, a first object of the present invention is toprovide a machine tool which has a structure for suppressing generationof errors, to thereby improve machining accuracy.

A second object of the present invention is to inexpensively provide abed for a machine tool which realizes low thermal displacement.

In order to achieve the first object, the present invention provides amachine tool, comprising: an X-axis moving unit, a Y-axis moving unit,and a Z-axis moving unit for producing relative movements between a tooland a workpiece along the respective directions of an X-axis, a Y-axis,and a Z-axis, which differ from one another; a C-axis drive unit forrotating the workpiece about a C-axis parallel to the Z-axis; and aB-axis turning unit for turning the tool about a B-axis which is definedon the B-axis turning unit and is parallel to the Y-axis. The tool isdisposed in such a manner that a machining point of the toolsubstantially coincides with the B-axis. The moving units, the driveunit, and the turning unit are controlled in such a manner that a workpoint of the workpiece substantially coincides with the machining pointof the tool.

In the machine tool of the present invention, the position of the toolis determined in such a manner that the machining point of the toolsubstantially coincides with the B-axis. Therefore, even when an erroris generated in turning movement of the B-axis turning unit, theposition of the machining point can be maintained on the B-axis, wherebyan error in the position of the machining point can be suppressed. Thisfeature effectively suppress an index error during B-axis turning, alongwith an error stemming from a positioning deviation at the time ofB-axis stoppage.

As described above, since the machine tool of the present invention hasa structure which hardly generates errors, machining accuracy can beimproved.

Preferably, the B-axis turning unit is disposed on the Y-axis movingunit in such a manner that the B-axis substantially coincides with acenter axis of a movable member of the Y-axis moving unit, the centeraxis extending along the Y-axis; and the tool is disposed on the B-axisturning unit.

In this case, the machining point of the tool can be located on thecenter axis of the movable member of the Y-axis moving unit. Therefore,when machining is performed while the Y-axis moving unit is driven tomove the movable member along the Y-axis direction, unnecessary stressesacting on drive means or the like can be suppressed, whereby errorscaused by inclination of the Y-axis moving unit and the B-axis turningunit can be suppressed. Moreover, since the B-axis turning unit carryingthe tool is disposed on the Y-axis moving unit whose error issuppressed, error in the position of the machining point of the tool canbe suppressed further.

Preferably, the C-axis drive is disposed on the Z-axis moving unit insuch a manner that the C-axis substantially coincides with a center axisof a movable member of the Z-axis moving unit, the center axis extendingalong the Z-axis.

In this case, the work point of the workpiece can be located in thevicinity of the center axis of the movable member of the Z-axis movingunit. Therefore, when the workpiece held by the C-axis drive unit ismachined, while the workpiece is moved along the Z-axis direction bymeans of the Z-axis moving unit in order to be pressed against the tool,unnecessary stresses which act, for example, on drive means for Z-axisdrive due to influence of the reaction of the pressing operation can besuppressed, whereby errors caused by inclination of the Z-axis movingunit and the C-axis turning unit can be suppressed.

Preferably, the machine tool has a bed having a horizontal top surfaceand a vertical side surface, wherein the X-axis moving unit is disposedon the horizontal top surface of the bed, the Z-axis moving unit isdisposed on the X-axis moving unit, and the C-axis drive unit isdisposed on the Z-axis moving unit, and wherein the Y-axis moving unitis disposed on the vertical side surface of the bed in such a mannerthat the Z-axis-direction center axis of the movable member of theZ-axis moving unit perpendicularly intersects the Y-axis-directioncenter axis of the movable member of the Y-axis moving unit, the B-axisturning unit is disposed on the Y-axis moving unit, and the tool isdisposed on the B-axis turning unit.

In this case, a bed having a complicated shape is not required, and abed having a substantially rectangular parallelepiped shape can be used.Therefore, the accuracy of the bed can be easily improved, and thus theindividual moving units, turning unit. etc. can be mounted on the bedwith improved positional accuracy.

In order to achieve the second object, the present invention provides abed structure for a machine tool, comprising: a bed formed throughcasting, the bed having a hollow structure and a hole as cast; and acover for covering the hole as cast in order to close the interior ofthe bed.

This structure decreases the area of a surface exposed to the outsideair, to thereby suppress total thermal displacement of the bed.

Preferably, a liquid is charged into the interior of the bed. In thiscase, since the heat capacity of the bed increases, thermal displacementcan be suppressed to a greater degree as compared to the case where thehole as cast is merely closed. Preferably, the liquid is oil, or watercontaining a rust preventing agent.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood by reference to the following detailed description ofthe preferred embodiments when considered in connection with theaccompanying drawings, in which:

FIGS. 1A to 1C are views showing a conventional machine tool;

FIG. 2A is a side view of a machine tool according a first embodiment ofthe present invention;

FIG. 2B is a cross-sectional view of the machine tool taken along lineIIB-IIB in FIG. 2A;

FIG. 2C is an enlarged side view showing the positional relation betweena workpiece and a tool;

FIG. 3A is a plan view of the machine tool;

FIG. 3B is a cross-sectional view of the machine tool taken along lineIIIB-IIIB in FIG. 3A;

FIG. 3C is an enlarged plan view showing the positional relation betweenthe workpiece and the tool;

FIG. 4 is a perspective view of the machine tool;

FIGS. 5A to 5C are explanatory views showing suppression of errors;

FIGS. 6A to 6C are explanatory views showing suppression of errors;

FIG. 7 is a cross sectional view of the bed taken along line VII-VII inFIG. 3A;

FIG. 8 is a cross sectional view of the bed taken along line VIII-VIIIin FIG. 2A;

FIG. 9 is an enlarged cross sectional view showing a hole as cast in aside wall of the bed, closed by a cover;

FIG. 10 is an enlarged cross sectional view showing a hole as cast in abottom wall of the bed, closed by a cover;

FIG. 11 is a table showing volumes V, surface areas S, ratios S/V,weights, and total heat capacities of modeled conventional bedstructures and molded bed structures of the present invention;

FIG. 12 is a graph showing the results of measurement of the interiortemperature (room temperature) of a thermostatic room and the interiortemperature of the bed of the machine tool in a state in which the castholes in the side walls and bottom wall of the bed are closed by meansof covers;

FIG. 13 is a graph showing the results of measurement of the interiortemperature (room temperature) of a thermostatic room and the liquidtemperature of the bed of the machine tool in a state in which the castholes in the side walls and bottom wall of the bed are closed by meansof covers, and the interior of the bed is filled with liquid; and

FIG. 14 is a graph showing results of calculation for obtainingtemperature changes of three objects which have the same volume and thesame temperature difference with respect to outside air temperature, buthave different surface areas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A machine tool according to a first embodiment of the present inventionwill now be described with reference to the drawings.

<Overall Structure>

The arrangement of individual moving units, a turning unit, etc. of themachine tool will be described with reference to FIGS. 2A to 2C, FIGS.3A to 3C, and FIG. 4; and the positional relation among the center axesof movable members of the individual moving units, the turning unit.etc. will be described with reference to FIGS. 2A to 2C and FIGS. 3A to3C. FIG. 2A shows a left-hand side view of the machine tool; FIG. 2B isa cross-sectional view of the machine tool taken along line IIB-IIB inFIG. 2A (a cross-sectional view of a Y-axis moving unit 30); and FIG. 2Cis an enlarged side view showing the positional relation between a workpoint A of a workpiece W and a machining point B of a tool T shown inFIG. 2A. FIG. 3A shows a plan view of the machine tool; FIG. 3B is across-sectional view of the machine tool taken along line IIIB-IIIB inFIG. 3A (a cross-sectional view of a Z-axis moving unit 50); and FIG. 3Cis an enlarged side view showing the positional relation between thework point A of the workpiece W and the machining point B of the tool Tshown in FIG. 3A.

The machine tool according to the present embodiment is a superprecision machine tool adapted to machine a workpiece, such as a lens ora lens mold, having an axisymmetric shape or a free curved surface withan accuracy of several hundreds to several tens of nanometers.

Various tools may be used as the tool T. For example, as shown in FIG.5A, a grinding wheel supported and rotated by a drive unit 10 may beused. Alternatively, as shown in FIG. 6A, a cutting tool (turning tool)may be used. In the example shown in FIGS. 2A to 4, the grinding wheelshown in FIG. 5A is used as the tool T. In this case, the machiningpoint B of the tool T is located on a circumferential surface of thetool T, and the machining point B of the tool T is brought into contactwith the work point A of the workpiece W to thereby grind the workpieceW. Notably, the position of the work point A may be changed on theworkpiece W.

The machine tool has a bed 1, which generally assumes the shape of arectangular parallelepiped. The bed 1 has a horizontal top surface(extending along the X-axis and Z-axis directions in FIGS. 2A and 3A),and vertical side surfaces (extending along the Y-axis direction inFIGS. 2A and 3A). Since the bed 1 has a rectangular parallelepipedshape, which is very simple, each surface can easily be machined to havehigh accuracy (in terms of the horizontalness of the horizontal surface,and the verticalness of the vertical surfaces). Individual parts, etc.,which affect machining accuracy, can be accurately disposed on thecorresponding surfaces, and position-adjusted thereon. Thus, themachining accuracy can be improved further.

An X-axis moving unit 60 is disposed on the top surface of the bed 1 inorder to produce relative movement between the work point A of theworkplace W and the machining point B of the tool T along a horizontaldirection (along the X-axis direction in FIGS. 2A to 4). As shown inFIG. 2A, the X-axis moving unit 60 includes a guide mechanism(stationary member) 60 a, a movable member 60 b, and a linear motor 60c. The movable member 60 b is in slidable engagement with the guidemechanism 60 a, and is reciprocated along the X-axis direction by meansof the linear motor 60 c.

In order to minimize position errors involved in linear motion, thelinear motor 60 c is used as drive means of the X-axis moving unit 60,instead of a motor of a rotary motion type. Therefore, a mechanism forconverting rotary motion to linear motion becomes unnecessary, themovable member can be moved directly along a straight path, and backlashis hardly generated, whereby errors can be reduced further.

The center axis of the movable member 60 b of the X-axis moving unit 60,which axis extends along the X-axis direction, is referred to as theX-axis center axis 60 z (see FIG. 2A).

The above-mentioned Z-axis moving unit 50 is disposed on the top surfaceof the X-axis moving unit 60 in order to produce a relative movementbetween the work point A of the workpiece W and the machining point B ofthe tool T along a horizontal direction perpendicular to the X-axis(along the Z-axis direction in FIGS. 2A to 4). As shown in FIG. 3B, theZ-axis moving unit 50 includes a guide mechanism (stationary member) 50a, a movable member 50 b, and a linear motor 50 c. The movable member 50b is in slidable engagement with the guide mechanism 50 a, and isreciprocated along the Z-axis direction by means of the linear motor 50c.

For the same reason as mentioned in connection with the X-axis movingunit 60, the linear motor 50 c is used as drive means of the Z-axismoving unit 50. The center axis of the movable member 50 b of the Z-axismoving unit 50, which axis extends along the Z-axis direction, isreferred to as the Z-axls center axis 50 z (see FIG. 3B).

Notably, the distance between the X-axis center axis 60 z and the Z-axiscenter axis 50 z is preferably reduced to a possible extent so as toreduce errors.

A C-axis drive unit 40 is disposed at the point of intersection betweenthe Z-axis center axis 50 z and a front end face of the movable member50 b of the Z-axis moving unit 50. The C-axis drive unit 40 supports theworkpiece W and rotates the same about a C-axis drive axis (C-axls;i.e., a horizontal direction which coincides with the Z-axis directionin FIGS. 2A and 3A), in order to produce a relative turn (rotation)between the work point A of the workpiece W and the machining point B ofthe tool T about the C-axis (in this case, the Z-axis).

The above-described Y-axis moving unit 30 is disposed on a side surfaceof the bed 1 in order to produce relative movement between the workpoint A of the workpiece W and the machining point B of the tool T alongthe vertical direction (along the Y-axis direction in FIGS. 2A to 4). Asshown in FIG. 2B, the Y-axis moving unit 30 includes a guide mechanism(stationary member) 30 a, a movable member 30 b, and a linear motor 30c. The movable member 30 b is in slidable engagement with the guidemechanism 30 a, and is reciprocated along the Y-axis direction by meansof the linear motor 30 c.

For the same reason as mentioned in connection with the X-axis movingunit 60, the linear motor 30 c is used as drive means of the Y-axismoving unit 30. The center axis of the movable member 30 b of the Y-axismoving unit 30, which axis extends along the Y-axis direction, isreferred to as the Y-axis center axis 30 z (see FIG. 2B).

Notably, a balance cylinder 80 is disposed under the Y-axis moving unit30 in order to support the movable member 30 b, on which is mounted aB-axis turning unit 20 carrying the tool T, with a force substantiallyequal to the force of gravity. This configuration reduces the loadacting on the linear motor of the Y-axis moving unit 30, so as tofurther reduce errors. Notably, the center axis of the balance cylinder80 extending along the Y-axis direction is adjusted to coincide with theY-axis center axis 30 z of the movable member 30 b, to thereby preventapplication of off-axis forces.

The above-mentioned B-axis turning unit 20 is disposed on the topsurface of the Y-axis moving unit 30 in order to produce a relative turnbetween the work point A of the workpiece W and the machining point B ofthe tool T about a B-axis turning axis (B-axis); i.e., a verticaldirection which coincides with the Y-axis direction in FIG. 2A). Asshown in FIG. 3A, a B-axis turntable 20 b is provided on the B-axisturning unit 20, and is turned about the B-axis. The tool T is fixedlydisposed on the B-axis turntable 20 b, whereby the direction of themachining point B of the tool T (orientation of the tool T within ahorizontal plane) can be changed or indexed.

The tool T is disposed on the B-axis turntable 20 b of the B-axisturning unit 20 in such a manner that the machining point B of the toolT on the circumferential surface thereof coincides with the B-axisturning axis (B-axis). Therefore, irrespective of angular position ofthe B-axis turntable 20 b, the machining point B of the tool T remainson the B-axis turning axis (B-axis) with substantially no deviationtherefrom. Notably, the orientation of the tool T at the machining pointB changes in accordance with the turn angle of the B-axis turntable 20b.

The B-axis turning unit 20 is disposed on the top surface of the Y-axismoving unit 30 in such a manner that the Y-axis center axis 30 zcoincides with the B-axis turning axis (B-axis). Further, the C-axisdrive unit 40 is disposed at the front end portion of the Z-axis movingunit 50 in such a manner that the Z-axis center axis 50 z coincides withthe C-axis drive axis (C-axis).

Notably, as shown in FIG. 4, the machine tool is equipped with amicroscope 90 for initial positioning of the work point A and themachining point B, and a stroboscope 92 for assisting the positionchecking by the microscope 90. Moreover, a fine adjustment mechanism 12is provided between the tool T and the B-axis turntable 20 b (tableturned by the B-axis turning unit 20) in order to attain a perfect matchbetween the machining point B of the tool T and the B-axis. A machineoperator operates the fine adjustment mechanism 12, while viewing themachining point B of the tool T by use of the microscope 90, in such amanner that the machining point B of the tool T coincides with theB-axis (in the example shown in FIG. 4, the operator finely adjusts theposition of the drive unit 10, which supports and drives the tool T).

Moreover, in FIG. 4, there is shown a shock-absorbing base 3, whichprecisely maintains the bed 1 in a horizontal posture with respect tothe floor surface, and absorbs vibrations from the floor surface or thelike.

<Suppression of Error in Turn Angle of B-Axis Turning Unit (FIGS. 5A to5C)>

Next, the reason why error in turn angle of the B-axis turning unit 20is suppressed will be described with reference to FIGS. 5A to 5C. In theconventional machine tool shown in FIG. 1B, because of the “distanceLbw” between the B-axis turning axis (B-axis) and the work point A ofthe workpiece W, an error in turn angle (error angle α) may affect theposition of the work point A. The error is at a level which can beignored in a machine tool which performs ordinary machining. However, ina precision machine tool which performs machining with very highaccuracy on the order of several hundreds to several tens of nanometers,influence of such error is large, and such error must be suppressed.

Such a positional error can be reduced by reducing the “distance Lbw” toa value near zero. However, since the work point A of the workpiece W isset at different positions on the workplace W, reducing the distance Lbwto a value near zero is considerably difficult (even when the distancebetween the B-axis and a certain work point is reduced to zero, thedistance between the B-axis and another work point does not becomezero). In view of the foregoing, in the present embodiment, instead ofthe work point A of the workpiece W, the machining point B of the tool Tis turned by means of the B-axis turning unit 20 (because the machiningpoint B of the tool T maintains a constant position).

In order to reduce the distance between the B-axis turning axis and themachining point B of the tool T to a value near zero, the tool T isdisposed as shown in FIG. 5B, whereby the machining point B of the toolT coincides with the B-axis turning axis. Therefore, even when an “errorangle α” is produced as shown in FIG. 5C when the B-axis turning unit 20is rotated by an angle θ from a position (indicated by broken lines) atwhich the C-axis is parallel to the tool T, an error is hardly generatedin the position “B” of the work point. As described above, the machinetool according to the present invention can effectively suppress anindex error during B-axis turning, along with an error stemming from apositioning deviation at the time of B-axis stoppage.

In the conventional machine tool shown in FIGS. 1A to 1C, since theC-axis unit 207 is mounted on the B-axis unit 209, the B-axis unit 209is large and heavy. In contrast, in the present embodiment, only thetool T and the drive unit 10 are mounted on the B-axis turning unit 20,so that the B-axis turning unit 20 can be reduced in size and weight.

<Suppression of Stress Generated Between Work Point of Workpiece andY-Axis Center Axis (FIGS. 6A to 6C)>

Next, the reason why stress generated between the work point A of theworkpiece W and the Y-axis center axis 30 z is suppressed will bedescribed with reference to FIGS. 6A to 6 c. Notably, in FIGS. 6A to 6C,the fine adjustment mechanism 12 shown in FIG. 4 is omitted.

In the conventional machine tool shown in FIG. 1C, because of the“distance Lyw” between the work point A of the workpiece W and theY-axis drive axis, an unnecessary stress is generated, and an “errorangle β may affect the position of the work point A. The error is at alevel which can be ignored in a machine tool which performs ordinarymachining. However, in a precision machine tool which performs machiningwith very high accuracy on the order of several hundreds to several tensof nanometers, influence of such error is large, and such error must besuppressed.

Such an unnecessary stress can be suppressed by reducing the distanceLyw” to a value near zero. In view of this, in the present embodiment,the B-axis turning axis (i.e., the machining point B of the tool T) ismade coincident with the Y-axis center axis 30 z in order to make thework point A of the workpiece W coincident with the Y-axis center axis30 z (reduce the distance therebetween to substantially zero), wherebygeneration of the error angle β as shown in FIG. 1C is suppressed.

<Suppression of Stress Generated Between C-Axis Drive Axis and Z-AxisCenter Axis>.

Next, the reason why stress generated between the C-axis drive axis andthe Z-axis center axis 50 z is suppressed will be described. In the casewhere the C-axis drive axis and the Z-axis center axis 50 z areseparated from each other, when the work point A of the workpiece W ismoved along the Z-axis direction by means of the Z-axis moving unit 50so as to press the work point A to the machining point B of the tool T,a stress is generated in the direction (in the example of FIG. 5B, theleft direction along the Z-axis) opposite the pressing direction (in theexample of FIG. 5B, the right direction along the Z-axis). In order tosuppress influence of the stress, the C-axis drive axis is madecoincident with the Z-axis center axis 50 z. Even in a case where thework point A of the workpiece W is not located on the C-axis drive axis,the distance between the work point A and the C-axis drive axis can bereduced (on average) to a possible extent, whereby generation of errorsstemming from unnecessary stress can be suppressed.

As described above, the machine tool of the present invention isconfigured in such a manner that the X-axis center axis 60 z, the Z-axiscenter axis 50 z, the C-axis drive axis (C-axis), the Y-axis center axis30 z, the B-axis turning axis (B-axis), and the machining point B of thetool T are located at proper positions, whereby generation of errors issuppressed and machining accuracy is improved.

The machine tool of the present invention is not limited to the details,such as structure and shape, described in the embodiment, and can besubjected to modification, addition, and deletion without departing fromthe scope of the invention.

The type of the tool T and the machining direction of the tool T are notlimited to those described in the embodiment. For example, the tool Tshown in FIGS. 6A to 6C and having a horizontally directed cutting edge(machining portion) may be replaced with a tool T having a verticallydirected cutting edge (machining portion).

Further, although in the embodiment the X-axis, Y-axis, and Z-axis areorthogonal coordinates, the X-axis, Y-axis, and Z-axis are notnecessarily required to intersect perpendicularly.

<Structure of Bed>

Next, the structure of the bed 1 will be described in detail withreference to FIGS. 7 to 10.

The bed 1 is formed through casting of iron, and as shown in FIGS. 7 and8, has a hollow, rib-reinforced inner structure. Specifically, ribs 10are integrally formed in the interior of the bed 1 in such a manner thatthe ribs 10 are arranged in a grid pattern in order to reinforce the bed1 and divide the interior of the bed 1 into twelve chambers which havethe same volume and are arranged in a matrix of 2 (longitudinaldirection)×3 (transverse direction)×2 (height direction). A through hole11 is formed in each of the ribs 10 in order to connect adjacentchambers.

Cast holes 102 are formed in the bottom wall of the bed 1 and in theside walls of the bed 1, except for the side wall to which the Y-axismoving unit 30 is attached. Therefore, no cast hole is formed in the topwall of the bed 1. The cast hole 102 is provided in order to removecasting sand from the individual chambers of the bed 1 after casting. Inorder to facilitate the removal of casting sand, each chamber isprovided with at least one cast hole 102. The cast holes 102 formed inthe side walls of the bed 1 are closed by means of covers 103, and thecast holes 102 formed in the bottom wall of the bed 1 are closed bymeans of covers 104, whereby the interior of the bed 1 is completelyclosed.

The covers 103 for closing the cast holes 102 formed in the side wallsof the bed 1 have a diameter greater than that of the cast holes 102, inorder to completely cover the cast holes 102. A hole for allowingpassage of a bolt 107, which will be described later, is formed in acentral portion of each cover 103. Further, as shown in FIG. 9, anannular groove is formed in a peripheral portion of each cover 103 toextend through the entire circumference, which portion comes into closecontact with the bed 1; and an O-ring 105 is fitted into the groove inorder to seal the interior of the bed 1. The cover 103 is fixed to thebed by means of a clamper 106 and the bolt 107. Specifically, theclamper 106 has a cruciform shape, and has a threaded hole at the centerthereof. The bolt 107 is passed through the cover 103 and is screwedinto the threaded hole of the clamper 106. When the bolt 107 is fastenedor screwed into the threaded hole, the clamper 106 comes into closecontact with the bed 1. Thus, the cover 103 comes in close contact withthe bed 1, and completely covers the cast hole 102, to thereby preventleakage of air from the interior of the bed 1 and entry of outside airinto the interior of the bed 1.

Meanwhile, the covers 104 for closing the cast holes 102 formed in thebottom wall of the bed 1 have a diameter greater than that of the castholes 102, in order to completely cover the cast holes 102. Each cover104 has a plurality of holes formed in a peripheral portion thereof.Further, as shown in FIG. 10, an annular groove is formed in aperipheral portion of each cover 104 to extend through the entirecircumference, which portion comes into close contact with the bed 1;and an O-ring 109 is fitted into the groove. Bolts 108 are passedthrough the holes of the cover 104 and then screwed into unillustratedthreaded portions of holes formed in the bed 1 around the correspondingcast hole 102. When the bolts 108 are fastened or screwed into thethreaded portions, the cover 104 comes into close contact with the bed1, and completely covers the cast hole 102, to thereby prevent leakageof air from the interior of the bed 1 and entry of outside air into theinterior of the bed 1.

As described above, since the cast holes 102 formed in the side wall andthe bottom wall of the bed 1 are closed by means of the covers 103 and104, the interior of the bed 1 becomes a closed space, and thus, thearea of the surface exposed to the outside air decreases, whereby thethermal displacement of the entire bed 1 can be suppressed. As a result,accuracy during long-time machining can be stabilized. Notably,reference numeral 112 denotes liquid charging openings to be used in asecond embodiment. The liquid charging openings 112 are unnecessary inthe first embodiment, and are closed by means of plugs.

Next, the second embodiment will be described. In the second embodiment,the cast holes 102 of the bed 1 are closed by use of covers, and aliquid is charged into the interior of the bed 1. Notably, the bed 1according to the second embodiment is identical in structure with thebed 1 according to the first embodiment. The process of fabricating thebed 1 is identical with the process employed in the first embodiment upto the point where the cast holes 102 are closed by use of the covers103 and 104. Subsequently, a liquid is charged into the closed interiorof the bed 1. Since the O-rings 105 and 109 are fitted to the covers 103and 104, respectively, the liquid does not leak through portions wherethe covers 103 and 104 are in close contact with the bed 1.

The liquid to be charged into the interior of the bed 1 is injected fromthe liquid charge openings 112 provided in the top wall of the bed 1. Ingeneral, plugs are fitted into the liquid charge openings 112 in orderto prevent entry of outside air. The plugs are removed from the liquidcharge openings 112 before injection of the liquid. After completion ofinjection of the liquid, the plugs are again fitted to the liquid chargeopenings 112 in order to prevent entry of outside air and evaporation ofthe liquid, which results in a reduction in the amount of the liquid.

Water, by virtue of its large specific heat, is most preferably used asthe liquid charged into the interior of the bed 1. Moreover, a rustpreventing agent is preferably added to water in order to avoid rustingof the bed 1 made of cast iron. Furthermore, ethylene glycol serving asan antifreezing fluid may be added to water so as to prevent freezing ofthe water. Instead of water, oil may be charged into the interior of thebed 1, thereby providing rust prevention and antifreeze protection.

As described above, the cast holes 102 of the bed 1 are closed by meansof the covers 103 and 104 so that the interior of the bed 1 becomes aclosed space; and a liquid is charged into the interior of the bed 1.Therefore, the thermal capacity of the entire bed increases, and thermaldisplacement of the entire bed can be suppressed to a greater extent ascompared with a bed whose cast holes are closed by means of covers, butwhose interior is not filled with liquid.

FIG. 11 shows a table which shows the relation among volume V, surfacearea S, ratio S/V, weight, and total heat capacity of modeledconventional bed structures and molded bed structures of the presentinvention. The table of FIG. 11 shows data for six bed structures; i.e.,a cast-iron bed A having a cubic solid structure (1 m×1 m×1 m); acast-iron bed B having a cubic hollow structure (1 m×1 m×1 m) whoseinterior is divided by ribs (thickness; 50 mm) into 27 chambers arrangedin a matrix of 3 (longitudinal direction)×3 (transverse direction)×3(height direction): a granite bed C having a cubic solid structure (1m×1 m×1 m): a bed Bo identical with the hollow, rib-reinforced,cast-iron bed B, except that cast holes are closed by means of covers; abed B1 identical with the bed Bo whose cast holes are closed by means ofcovers, except that mineral oil is charged into the interior of the bed;and a bed B2 identical with the bed Bo whose cast holes are closed bymeans of covers, except that water is charged into the interior of thebed.

First, the cast-iron bed A having a solid structure has advantageousfeatures, such as small surface area and large heat capacity. However,as described above, the cast-iron bed A having a solid structure is notpreferable, from the viewpoint of weight and influence of distortioncaused by long-term changes. Therefore, a conventional cast-iron bed isfabricated to have a hollow, reinforced structure, as the bed B, tothereby remove about 70% of the cast iron. When the hollow, cast-ironbed B is compared with the granite bed C having a solid structure, thegranite bed C has a larger heat capacity and a smaller ratio (S/V) ofsurface area S to volume V. Therefore, the granite bed C can be said tobe a structure which is less likely to follow changes in the outside airtemperature.

However, in the case of the bed Bo identical with the hollow,rib-reinforced, cast-iron bed B, except that cast holes are closed bymeans of covers, since the area of a surface in contact with the outsideair decreases by virtue of closure of the cast holes by covers, ascompared with the bed B the bed Bo has a reduced ratio S/V, and is lesslikely to follow changes in the outside air temperature. Moreover, inthe case of the bed B1 identical with the bed Bo whose cast holes areclosed by means of covers, except that mineral oil is charged into theinterior of the bed, a heat capacity almost the same as that of thegranite bed C is obtained; and in the case of the bed B2 filled withwater, a heat capacity two times that of the granite bed C is obtained.Therefore, these beds B1 and B2 are much less likely to follow changesin the outside air temperature, or are unresponsive to changes in theoutside air temperature.

FIG. 12 is a graph showing the results of measurement of the interiortemperature (room temperature) of a thermostatic room and the interiortemperature of the bed 1 according to the first embodiment in which thecast holes 102 in the side walls and bottom wall of the bed 1 are closedby means of the covers 103 and 104. The interior temperature of thethermostatic room is set to 20° C. The interior temperature of thethermostatic room and the interior temperature of the bed were measuredby use of platinum thermometer resistors. Measurement of the interiortemperature of the bed was performed by use of a platinum thermometerresistor inserted into the liquid charging opening 112 of the bed 1. Ascan be seen from FIG. 12, when the machine tool is started ({circlearound (1)} in FIG. 12), the interior temperature of the thermostaticroom increases, because of heat generation of the machine tool, andfluctuates because of disturbances such as entry of a person into thethermostatic room and departure of the person therefrom. When themachine tool is stopped ({circle around (2)} in FIG. 12), the interiortemperature of the thermostatic room decreases to the vicinity of theset temperature, because no heat is generated from the machine tool.

The interior temperature of the bed also increases when the machine toolis started. However, the interior temperature of the bed does notcoincide with the interior temperature of the thermostatic room, andslowly increases with the interior temperature of the thermostatic room.Further, the graph demonstrates that the interior temperature of the bedis hardly influenced by changes in the interior temperature of thethermostatic room.

In other words, since the interior of the bed 1 is completely closed byclosing the cast holes 102 in the side walls and bottom wall of the bed1 by means of the covers 103 and 104, the interior temperature of thebed 1 becomes less likely to follow changes in the interior temperatureof the thermostatic room; i.e., becomes comparatively unresponsive tochanges in the interior temperature of the thermostatic room. As aresult, although the outside surfaces of the bed 1 receive the influenceof changes in the interior temperature of the thermostatic room, theinside surfaces of the bed 1 hardly receive the influence of changes inthe interior temperature of the thermostatic room. Therefore, the areaof a surface of the bed which undergoes changes in the outside airtemperature decreases, and the thermal displacement of the entire bedcan be suppressed.

FIG. 13 is a graph showing the results of measurement of the interiortemperature (room temperature) of a thermostatic room and the liquidtemperature of the bed 1 according to the second embodiment in which aliquid is charged into the interior of the bed 1. The interiortemperature of the thermostatic room is set to 20° C. Water containing arust preventing agent was used as the liquid charged into the interiorof the bed 1. The interior temperature of the thermostatic room and theliquid temperature were measured by use of platinum thermometerresistors. Measurement of the liquid temperature was performed by use ofa platinum thermometer resistor inserted into the liquid chargingopening 112 of the bed 1. As can be seen from FIG. 13, as in the caseshown in FIG. 12, when the machine tool is started ({circle around (1)}in FIG. 13), the interior temperature of the thermostatic room increasesbecause of heat generation of the machine tool, and fluctuates becauseof disturbances such as entry of a person into the thermostatic room anddeparture of the person therefrom. When the machine tool is stopped({circle around (2)} in FIG. 13), the interior temperature of thethermostatic room decreases to the vicinity of the set temperature,because no heat is generated from the machine tool.

The liquid temperature also increases when the machine tool is started.However, the liquid temperature does not coincide with the interiortemperature of the thermostatic room, and slowly increases with theinterior temperature of the thermostatic room. Further, the graphdemonstrates that the liquid temperature is hardly influenced by changesin the interior temperature of the thermostatic room.

When the bed 1 according to the second embodiment is compare with thebed 1 according to the first embodiment in which the cast holes 102 inthe side walls and bottom wall of the bed 1 are closed by means of thecovers 103 and 104, but no liquid is charged into the interior of thebed 1, the internal temperature of the bed 1 according to the secondembodiment filled with liquid becomes much less likely to follow changesin the interior temperature of the thermostatic room; i.e., becomescomparatively unresponsive to changes in the interior temperature of thethermostatic room. In other words, since the total heat capacity of thebed increases by virtue of water containing a rust-preventing-agent andcharged into the interior of the bed 1, the bed 1 according to thesecond embodiment can be said to become much less likely to followchanges in the interior temperature of the thermostatic room, or tobecome comparatively unresponsive to changes in the interior temperatureof the thermostatic room, as compared with the case where the cast holesof the bed are merely closed by means of covers. As a result, thethermal displacement of the entire bed can be suppressed to a greaterextent, as compared with the case where the cast holes of the bed aremerely closed by means of covers.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, thepresent invention may be practiced otherwise than as specificallydescribed herein.

1-7. (canceled)
 8. A bed structure for a machine tool, comprising: a bedformed through casting, the bed having a hollow structure and at leastone hole as cast; and a cover for covering each said at least one holeas cast in order to close the interior of the bed, wherein said coversubstantially prevents the passage of air.
 9. A bed structure for amachine tool, comprising: a bed formed through casting, the bed having ahollow structure and a hole as cast; and a cover for covering the holeas cast in order to close the interior of the bed, wherein a liquid ischarged into the interior of the bed.
 10. A bed structure according toclaim 9, wherein the liquid is oil, or water containing a rustpreventing agent.